1. Field of Invention
This disclosure relates to the field of power amplifier circuits, and more specifically to the detection and correction of saturation in power amplification circuits.
2. Discussion of Related Art
In some applications where power amplification of signals is required, precise control of the power gain may be desirable to achieve desired signal properties. For example, transmission modules used in communications devices such as cellular telephones, personal digital assistants (PDAs), etc. may require precise internal power control in certain modes of operation, such as Gaussian Minimum Shift Keying (GMSK) mode. In such applications, a power control circuit may be used that controls the gain of an amplifier stage. A typical control loop controls the amplifier gain via a loop error voltage based upon the difference between the output of a RF detector and a loop set point. The detector (which may be either a linear or a logarithmic detector) samples the amplified signal and produces a detector output indicative of the magnitude of the amplified signal (e.g., based upon the rf amplitude or the power of the amplified signal). The loop error voltage typically passes through an error amplifier (which may be proportional, integral, derivative, or a combination of any such elements, according to the requirements of the control loop design), yielding a gain control signal that controls the gain of the power amplifier.
Such power control feedback loops suffer severe degradation of performance as the amplifier approaches saturation. When the power amplifier saturates, increases in the gain control signal no longer result in increases in the power amplifier output. This leads to breakdown of the loop performance, such as, for example, the gain control voltage being pinned to the high rail as it increases in an attempt increase the output of the saturated power amplifier output. This condition is sometimes referred to as “loop saturation.”
In some applications, loop performance can be exceptionally sensitive to saturation, resulting in a power amplifier output other than what is desired. For example, in a typical circuit, control loop performance can degrade beyond acceptable limits at as little as 0.1 dB power amplifier saturation. One way to avoid loop saturation is to monitor the loop error signal and reduce the loop setpoint when saturation (or the imminent onset of saturation) is detected. Saturation can be difficult to detect, however, where the error induced by the saturation is small. In a typical loop circuit using a logarithmic detector to measure the amplifier output, for example, the detector sensitivity may be 50 mV per dB. An error of 0.1 dB in power then results in only 5 mV of error in the loop feedback signal. Since 5 mV is on the order of the error in standard CMOS amplifier input offsets, it may not be possible for the system to cleanly distinguish loop saturation from normal production variation in the performance of the amplifier itself.
Loop saturation may be easier to detect using a linear detector, where the detector sensitivity near saturation may be considerably higher; saturation can be observed directly by monitoring the loop error signal for deviation from zero when saturation occurs. However, as discussed further below, in a circuit using linear detection, applying a constant reduction to the loop setpoint results in an unacceptable distortion of the loop output. Further, in some applications it may be preferable for other reasons to use logarithmic detection. For example, compared to linear detection, logarithmic detection can provide a much wider dynamic range, which is desirable in many applications.
Thus, in many applications, it is preferable to use a logarithmic detector in the control loop, making saturation more difficult to detect.